Apparatus and method for determining the extent of ablation

ABSTRACT

A catheter for use in an electrophysiological procedure to ablate a site includes a first electrode having a first work function and energized by a source of RF energy. A second electrode having a second work function is disposed adjacent tissue. The difference in work functions of the first and second electrodes, operating in the presence of an electrolyte represented by intermediate tissue, produces an exchange of electrical charges through chemical reaction to create a galvanic cell. The galvanic cell serves as a current source to provide a unique electrode signal used to regulate the RF energy applied to the first electrode.

This is a continuation of application Ser. No. 08/851,879, filed May 6,1997, now U.S. Pat. No. 5,868,737 entitled "APPARATUS AND METHOD FORDETERMINING ABLATION", which is a continuation in part of applicationSer. No. 08/488,887 filed Jun. 9, 1995, now U.S. Pat. No. 5,697,925,entitled "APPARATUS AND METHOD FOR THERMAL ABLATION" and the presentapplication also claims priority with respect to common subject matterto provisional application Ser. No. 60/016,647 filed May 15, 1996,entitled "APPARATUS AND METHOD FOR INDICATING THERMAL ABLATION", all ofwhich applications are assigned to the present Assignee and describeinventions by the present inventors.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to catheters and, more particularly,temperature controlled catheter probes for ablating tissue.

2. Background of the Invention

The heart is a four chamber muscular organ (myocardium) that pumps bloodthrough various conduits to and from all parts of the body. In orderthat the blood be moved in the cardiovascular system in an orderlymanner, it is necessary that the heart muscles contract and relax in anorderly sequence and that the valves of the system open and close atproper times during the cycle. Specialized conduction pathways conveyelectrical impulses swiftly to the entire cardiac muscle. In response tothe impulses, the muscle contracts first at the top of the heart andfollows thereafter to the bottom of the heart. As contraction begins,oxygen depleted venous blood is squeezed out of the right atrium (one oftwo small upper chambers) and into the larger right ventricle below. Theright ventricle ejects the blood into the pulmonary circulation, whichresupplies oxygen and delivers the blood to the left side of the heart.In parallel with the events on the right side, the heart muscle pumpsnewly oxygenated blood from the left atrium into the left ventricle andfrom there out to the aorta which distributes the blood to every part ofthe body. The signals giving rise to these machinations emanates from acluster of conduction tissue cells collectively known as the sinoatrial(SA) node. The sinoatrial node, located at the top of the atrium,establishes the tempo of the heartbeat. Hence, it is often referred toas the cardiac pacemaker. It sets the tempo simply because it issuesimpulses more frequently than do other cardiac regions. Although thesinoatrial node can respond to signals from outside the heart, itusually becomes active spontaneously. From the sinoatrial node impulsesrace to the atrioventricular (AV) node above the ventricles and speedsalong the septum to the bottom of the heart and up along its sides. Theimpulses also migrate from conduction fibers across the overlying musclefrom the endocardium to the epicardium to trigger contractions thatforce blood through the heart and into the arterial circulation. Thespread of electricity through a healthy heart gives rise to the familiarelectrocardiogram. Defective or diseased cells are electricallyabnormal. That is, they may conduct impulses unusually slowly or firewhen they would typically be silent. These diseased cells or areas mightperturb smooth signalling by forming a reentrant circuit in the muscle.Such a circuit is a pathway of electrical conduction through whichimpulses can cycle repeatedly without dying out. The resulting impulsescan provoke sustained ventricular tachycardia: excessively rapid pumpingby the ventricles. Tachycardia dysrhythmia may impose substantial riskto a patient because a diseased heart cannot usually tolerate rapidrates for extensive periods. Such rapid rates may cause hypotension andheart failure. Where there is an underlying cardiac disease, tachycardiacan degenerate into a more serious ventricular dysrhythmia, such asfibrillation. By eliminating a reentrant circuit or signal pathwaycontributing to tachycardia, the source of errant electrical impulseswill be eliminated. Ablation of the site attendant such a pathway willeliminate the source of errant impulses and the resulting arrhythmia.Mapping techniques for locating each of such sites that may be presentare well known and are presently used.

Interruption of the errant electrical impulses is generally achieved byablating the appropriate site. Such ablation has been performed bylasers. The most common technique used at an ablation site involves theuse of a probe energized by radio frequency radiation (RF). Measurementand control of the applied RF energy is through a thermistor (or itcould be a thermocouple) located proximate the RF element at the tip ofa catheter probe. While such a thermistor may be sufficiently accurateto reflect the temperature of the thermistor, it is inherentlyinaccurate in determining the temperature of the tissue at the ablationsite. This results from several causes. First, there is a temperatureloss across the interface between the ablation site (usually variabledue to position of electrode) and the surface of the RF tip. Second, theflow of blood about the non-tissue contact portion of the conductive RFtip draws off heat from the ablation site which causes the thermistor tobe cooler than the tissue under ablation. However, temperatures above100° C. causes coagulum formation on the RF tip, a rapid rise inelectrical impedance of the RF tip, and excessive damage to theendocardium. Third, there is a lag in thermal conduction between the RFtip and the thermistor, which lag is a function of materials, distance,and temperature differential. Each of these variables may changeconstantly during an ablation procedure.

To ensure that the ablation site tissue is subjected to heat sufficientto raise its temperature to perform irreversible tissue damage, thepower transmitted to the RF tip must be increased significantly greaterthan that desired for the ablation in view of the variable losses. Dueto the errors of the catheter/thermistor temperature sensing systems,there is a propensity to overheat the ablation site tissue needlessly.This creates three potentially injurious conditions. First, the RF tipmay become coagulated. Second, tissue at the ablation site may "stickto" the RF tip and result in tearing of the tissue upon removal of theprobe. This condition is particularly dangerous when the ablation siteis on a thin wall of tissue. Third, inadequate tissue temperaturecontrol can result in unnecessary injury to the heart includingimmediate or subsequent perforation.

When radio frequency current is conducted through tissue, as might occurduring a procedure of ablating a tissue site on the interior wall(endocardium) of the heart with a radio frequency energized catheter,heating occurs preliminarily at the myocardial tissue interface with thetip of the catheter. Given a fixed power level and geometry of thecatheter probe, the temperature gradient from the probe interface and adistance, r, into the tissue is proportional to 1/r⁴. Heating is causedby the resistive (OHMIC) property of the myocardial tissue and it isdirectly proportional to the current density. As may be expected, thehighest temperature occurs at the ablation site which is at theinterface of the RF tip and the tissue.

When the temperature of the tissue at the ablation site approaches 100°C., a deposit is formed on the RF tip that will restrict the electricalconducting surface of the RF tip. The input impedance to the RF tip willincrease. Were the power level maintained constant, the interfacecurrent density would increase and eventually carbonization would occur.At these relatively extreme temperatures, the RF tip will often stick tothe surface of the tissue and may tear the tissue when the RF tip isremoved from the ablation site.

To effect ablation, or render the tissue nonviable, the tissuetemperature must exceed 50° C. If the parameters of the RF tip of acatheter are held constant, the size and depth of the lesion caused bythe ablation is directly proportional to the temperature and time at theinterface (assuming a time constant sufficient for thermal equilibrium).In order to produce lesions of greatest depth without overheating thetissues at the interface, critical temperature measurement techniques ofthe RF tip are required.

The current technology for measuring the temperature of an RF tipembodies a miniature thermistor(s) located in the RF tip of the probe.The present state of the art provides inadequate compensation for thethermal resistance that exists between the thermistor and the outersurface of the RF tip, which may be in variable contact with the tissueand affected by blood cooling or between the outer surface of the RF tipand the surface of the adjacent tissue. Because of these uncertaintiescontributing to a determination of the specific temperature of thetissue at the interface, apparatus for accurately determining whenablation actually occurs would be of great advantage in performing anelectrophysiological procedure to ablate a specific site(s) of themyocardial tissue.

SUMMARY OF THE INVENTION

A catheter probe having a metal tip energized by an RF generatorradiates RF energy as a function of the RF energy applied. When the tipis placed adjacent tissue at an ablation site, the irradiating RF energyheats the tissue due to the ohmically resistive property of the tissue.The catheter tip placed adjacent the ablation site on tissue incombination with an electrically conducting dissimilar metal plate incontact with tissue at a location remote from the ablation site and anelectrolyte defined by the intervening tissue create a galvanic cellwhen the tip and plate have different work functions because ofmigration of electrical charges therebetween. By loading the galvaniccell, the DC output current is a linear function of the temperature ofthe ablation site heated by the RF energy. The DC output current of thegalvanic cell is used to regulate the output of the RF generator appliedto the catheter tip to control the current density at the ablation site.When ablation at the ablation site occurs, the value of the DC outputsignal drops dramatically irrespective of further applied RF energy andprovides a signal to terminate application of further RF energy to avoidpossible coagulation of the RF tip, sticking of the tissue to the RF tipand perforation of the tissue.

It is therefore a primary object of the present invention to provide asignal for indicating occurrence of tissue damage at an ablation site inresponse to RF energy radiated from a catheter tip during an ablationprocedure.

Another object of the present invention is to provide an output signalrepresentative of the occurrence of tissue damage at an ablation sitefor subsequently regulating the RF radiation power level of a probeperforming the ablation procedure to obtain tissue damage to a desireddepth.

Yet another object of the present invention is to generate a signalrepresentative of actual tissue damage at an ablation site in order tocease further heating of the ablation site by regulating the radiationof RF energy from an ablating RF tip.

Still another object of the present invention is to provide apparatusfor determining the occurrence of tissue damage of a cardiac impulsepathway and thereafter cease further heating of the ablation site.

A further object of the present invention is to provide aself-regulating catheter mounted RF radiating element controlled by anoutput signal reflective of actual tissue damage at an ablation site onthe endocardium of a heart suffering tachycardia dysrhythmia and destroya pathway of errant electrical impulses at least partly contributing tothe tachycardia dysrhythmia.

A still further object of the present invention is to provide a methodfor controlling heating and sensing the occurrence of actual tissuedamage at an ablation site and thereafter terminating further heating ofthe ablation site when the desired depth of tissue damage has beenachieved.

These and other objects of the present invention will become apparent tothose skilled in the art as the description thereof proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be described with greater specificity andclarity with reference to the following drawings, in which:

FIG. 1 illustrates a simplified representation of the present invention;

FIG. 2 illustrates the current density at an ablation site during anablation procedure;

FIG. 3 illustrates a representation of a catheter probe embodying athermistor useful in the present invention;

FIG. 4 illustrates representatives curves for calibrating thetemperature of an ablation site through use of a probe embodying athermistor;

FIG. 5 is a block diagram of circuitry representatively shown in FIG. 1;

FIG. 6 illustrates a catheter probe for sequentially mapping theendocardium, identifying a site to be ablated and ablating the sitewithout relocating the probe;

FIGS. 7A and 7B are graphs illustrating the respective output signals ofthe power level applied by a catheter tip, the temperature sensed by acatheter mounted thermistor and the galvanic current at an ablation siteduring an ablation procedure; and

FIG. 8 illustrates the use of a computer to perform certain of thefunctions manually performed with the circuitry shown in FIG. 5 and toprovide displays of information.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Two electrodes of different metals having different work functions inthe presence of an electrolyte (such as blood) a saline solution orliving tissue, will produce an exchange of 5 electrical charges and anelectromotive force (emf) is generated. This emf generator is known as agalvanic cell. A technical discussion of the history of galvanic cellsis set forth in Chapter 1.3, entitled "Basic Electrochemistry" (pages12-31) in a textbook entitled Modern Electrochemistry, authored by JohnO'M. Bockris, published by Plenum Press., New York, dated 1970. Detailedtechnical discussions of galvanic cells can be found in: Chapter 4,entitled "Reversible Electrode Potentials" (pages 73-100) of a textbookentitled Electrochemistry Principles and Applications, authored byEdmund C. Potter, published by Cleaver-Hume Press, Ltd., dated 1956;Chapter 4 entitled "Electrodes and Electrochemical Cells" (pages 59-89)of a textbook entitled Introduction to Electrochemistry, authored by D.Bryan Hibbert, published by MacMillan Press Ltd., dated 1993; andChapter 12 entitled "Reversible Cells" (pages 282-311) of a textbookentitled Electrochemistry of Solutions, authored by S. Glasstone,published by Methuen & Co. Ltd., London, dated 1937 (Second Edition).These technical discussions are incorporated herein by reference.

The magnitude of the potential of a galvanic cell is a function of theelectrolyte concentrates and the metals' work functions. The opencircuit voltage of the galvanic cell is essentially constant despitetemperature changes at the interface between the electrodes and theelectrolyte. However, by loading the galvanic cell with a fixed valueshunt resistance it simulates a current generator which has an outputsignal directly proportional to the temperature of the metal andelectrolyte interface. The output signal of the current generator can becalibrated as a function of the temperature at the interface. A simplemethod for calibration is that of referencing the output of the currentgenerator with the output of a thermistor embedded in the electrode atsteady state power and temperature conditions at an initial or firsttemperature and at a second temperature. This will provide two datapoints for the power/temperature curve of the current generator. Sincethe output of the current generator is linear, the curve can be extendedto include all temperatures of interest.

The present invention is directed to apparatus for ablating an errantcardiac conduction pathway responsible for or contributing to arrhythmiaof the heart. The ablation process is performed by heating the ablationsite tissue to a temperature typically exceeding 50° C., sufficient tocause ablation of the cells contributing to the errant impulse pathway.The ablation is effected by irradiating the ablation site tissue withradio frequency (RF) energy. For this purpose, a catheter probe tip ispositioned adjacent the ablation site, which site has been previouslydetermined by mapping procedures well known to physicians and thoseskilled in the art. Upon positioning of the probe tip at the ablationsite, a source of RF energy is actuated to transmit RF energy through aconductor to the tip of the probe. The RF energy radiates from the tipinto the ablation site tissue. The current density at the ablation siteis a function of the power of the RF energy irradiating the ablationsite and the surface area defining the interface between the tip and theablation site tissue. Control of the tissue temperature at the interfaceis of significant importance to control the area and depth of ablationin order to perform the degree of ablation necessary, to preventcoagulation on the tip, to prevent the tip from sticking to the tissue,to prevent avoidable injury to adjacent tissue, to prevent perforationof the tissue, and to avoid unnecessary heating of the blood flowing inand about the tip.

Catheter probes having a thermistor embedded at the tip have been usedto perform an ablation procedure and the amount of RF energy applied hasbeen regulated as a function of the temperature sensed by thethermistor. Such temperature sensing is inherently inaccurate indetermining the temperature at the ablation site due to the numerousvariables present. First, there exists a temperature loss through theinterface between the ablation site and the surface area of the tip incontact with tissue. Second, there exists a thermal resistance withinthe tip which causes temperature lag between the surface area of the tipin contact with the ablation site and the thermistor. Third, theorientation of the tip with respect to the ablation site will vary witha consequent variation of heating of the ablation site. Finally, theblood flowing about the tip area not in tissue contact will draw offheat as a function of both flow rate and orientation of the tip withrespect thereto. By experiment, it has been learned that the differencesbetween the tissue temperature at the ablation site and the temperatureregistered by a thermistor may range from 10° C. to 35° C. Suchtemperature excursion may result in unnecessary injury without aphysician being aware of the injury caused at the time of the ablationprocedure. Where ablation is being performed upon a thin wallmyocardium, a puncture or a perforation at a later time can and doesoccur with potentially disastrous results.

The present invention is shown in simplified format in FIG. 1. An RFgenerator 10 serves as a source of RF energy. The output of the RFgenerator is controlled by an input signal identified as J₁. The RFenergy, as controlled by J₁, is transmitted through a conductor 12 to acatheter probe 14. This probe is depicted as being lodged within a bloodfilled chamber 16 of a heart. The chamber may be the right or leftatrium or the right or left ventricle. Probe 14 is lodged adjacent, forinstance, tissue 18 at an ablation site 20 representing a reentrantcircuit to be ablated. As represented, blood continually flows throughchamber 16 about and around probe 14.

Probe 14 includes a tip 30 electrically connected to conductor 12 toirradiate ablation site 20 with RF energy.

Typically, the frequency may be in the range of about 350 kHz to about1200 kHz. Such irradiation of the ablation site will result in heatingof the ablation site as a function of the current density at theablation site. The current density is determined by the energy level ofthe irradiating RF energy and the surface area of the ablation site.More specifically, the heat generated is proportional to the currentdensity squared. This may be expressed as: T(r)=kPd=kI² R=(J_(O) ² /r⁴)R, where T=temperature, r=distance from the interface, J_(O) =currentdensity at the interface, Pd=power dissipated, I=current at theinterface, and R=resistance at the interface. The return path to RFgenerator 10 is represented by conductor 32. Conductor 32 iselectrically connected to a relatively large sized plate 34 placedadjacent the patient's skin, preferably a large surface area of thepatient's back. To ensure good electrical contact, an electricallyconducting salve may be disposed intermediate plate 34 and patient'sback 36. The fluid and tissues of the patient intermediate tip 30 andplate 34, represented by numeral 38, constitutes, in combination, anelectrolyte and therefore an electrically conductive path between thetip and the plate. The DC current flow is represented by i_(s) and theDC voltage is represented by v_(S).

As more particularly illustrated in FIG. 2, ablation site 20 has arelatively high concentration of current paths, representativelydepicted by diverging lines identified with numerals 42, 44, 46, 48, 50,and 52. These current paths are in close proximity with one another atthe ablation site. The resulting high current density will produceheating of the ablation site as a function of the current density. Thedepth of the ablated tissue is representatively illustrated by line 54.The current density proximate back 36 of the patient adjacent plate 34is relatively low. With such low current density, essentially no heatingof the skin adjacent plate 34 will occur. It is to be appreciated thatFIG. 2 is not drawn to scale and is intended to be solely representativeof relative current densities resulting from irradiation of an ablationsite by tip 30.

Ablation with tissue temperature control permits the physician tooptimize the ablation process by allowing the ablation to occur atmaximum temperature that is below a temperature conducive to formationof coagulation on the tip. Since such temperature is a function of theRF energy irradiating the ablation site tissue, control of the amount ofRF energy transmitted via conductor 12 to the tip is necessary. Apresently available type of catheter probe 60 is illustrated in FIG. 3.This probe includes a tip 62 for radiating RF energy received throughconductor 64 from a source of RF energy. A thermistor 66 is embedded intip 62 or in sufficient proximity with the tip to be responsive to thetemperature of the tip. A pair of conductors 68 and 70 interconnectthermistor 66 with a signal detection circuit to provide an outputsignal representative of the temperature sensed. Furthermore, probe 60may include mapping electrodes 72, 74 and 76. These electrodes may beused in conjunction with manipulation of probe 60 within the heart todetect and identify errant impulse pathways causing cardiac arrhythmia.Conductors 78, 80 and 82 connect electrodes 72, 74 and 76, respectively,to circuitry associated with the mapping functions, as is well known.

As stated above, thermistor 66 is incapable of providing an accuraterepresentation of the temperature at the ablation site. In summary, thecauses contributing to inaccurate temperature representation are heatloss through the interface between tip 30 and ablation site 20 (see FIG.2), thermal lag between the area of tissue in contact with the tip andthe sensing element of the thermistor, and heat loss resulting from flowof blood about the tip area not in contact with the tissue.

By experimentation, it has been learned that the combination of tip 30,plate 34 and body 38 perform in the manner of a galvanic cell providedthat the tip and the plate are metallic and of different work functionssince body 38 acts as an electrolyte; the body is permeated by fluidshaving electrical properties similar to a saline solution. Experimentsindicate that a preferable material for tip 30 is platinum and apreferable material for plate 34 is copper. The open circuit voltage(v_(S)) of this galvanic cell is essentially independent of thetemperature of ablation site 20. However, if the galvanic cell isheavily loaded with a shunt resistor, the galvanic cell serves as acurrent source and the magnitude of the current (i_(S)) is linear as afunction of the tissue temperature at the ablation site through the 37°C. to 100° C. temperature range of interest. The temperature of thetissue adjacent plate 34 is the body temperature since the currentdensity is insufficient to generate heat of any consequence. Thus, thegalvanic cell created by the apparatus illustrated in FIG. 2 provides anoutput signal representative of the tissue temperature at ablation site20 and irrespective of the temperature of tip 30.

One method for calibrating the galvanic cell will be described, butother methods may be used which do not require the presence of athermistor at the tip. A thermistor is embedded in the tip of a catheterprobe, such as probe 60. For reasons set forth above, the output of thethermistor is inherently inaccurate with respect to the actual tissuetemperature at the ablation site; moreover, the temperature sensed bythe thermistor as a function of the power applied is generallynonlinear. However, within a temperature range from a quiescent standbystate to a small temperature increase at the ablation site (smallincrease in power applied), the output signal of the thermistor isessentially linear. By matching the output curve of the thermistor withthe generally linear response curve of the galvanic cell, two coincidentreference points can be determined. Referring to FIG. 4, there isillustrated a thermistor response curve and a galvanic cell responsecurve manipulated to be coincident from a point 0 to a point 1. Bycorrelating the temperature indication of the thermistor at these twopoints, with the current output (i_(S)) of the galvanic cell, thetemperature response can be linearly extrapolated to obtain atemperature reading correlated with the current output of the galvaniccell. That is, for any given current output of the galvanic cell, thetissue temperature of the ablation site can be determined. Thus, ifprobe 14 illustrated in FIGS. 1 and 2 is of the type shown in FIG. 3,calibration of the probe at the ablation site can be readily determined.Other methods for calibrating the current output with temperature canalso be employed, as set forth above.

Referring to FIG. 5, there is illustrated a block diagram of the majorcomponents necessary to control the power applied to a catheter probefor ablating an errant impulse pathway at an ablation site. FIG. 5 showsa temperature input circuit 90 for setting a reference voltageequivalent to the tissue temperature sought for an ablation site atwhich an ablation procedure is to be performed. The resulting outputsignal is transmitted through conductor 92 to a servo amplifier 94. Theservo amplifier provides an output signal on conductor 96 to control theoutput power of RF generator 98. A switch 100 controls operation of theRF generator. The RF energy output is impressed upon conductor 102. Ablocking capacitor 104 is representative of a high pass filter andblocks any DC component of the signal on conductor 102. Conductor 106interconnects the blocking capacitor with tip 30 of probe 14 andtransmits RF energy to the tip. Tip 30 irradiates ablation site 20 of anendocardium, wall, membrane, or other living tissue to be irradiatedwith RF energy. Tip 30 is of a substance, such as platinum or othermetal, having a first work function. Plate 34 displaced from tip 30, isof a substance, such as copper or other metal, having a second workfunction which is different from the first work function. Plate 34 is inelectrical contact with a mass of tissue 38 intermediate tip 30 and theplate. This tissue, being essentially a liquid and having electricalcharacteristics of a saline solution, serves in the manner of anelectrolyte interconnecting tip 30 and plate 34. The resulting galvaniccell formed, as discussed above, provides a DC output voltage v_(S)across conductors 106 and 108. Shunt impedance R1 heavily loads thegalvanic cell formed to convert the galvanic cell to a current source(i_(S)) to provide an output signal reflective of the tissue temperatureat ablation site 20. The output signal from the galvanic cell istransmitted through conductor 110 to a lowpass filter 112. The output ofthe lowpass filter is conveyed via conductor 114 to an operationalamplifier 120 of a calibration circuit 116. Additionally, a signalmeasurement and processing circuit 118, connected to conductor 102through conductor 103 to provide sampling of the output load voltage(V_(O)). It is also connected to conductor 107 through conductor 105 toprovide an input signal of the load output) current (I_(O)) sensed,processes the input signals to 0provide an indication of the impedance,power, and voltage and current levels. A readout 123, connected throughconductor 119 to signal measurement and processing circuit 118, provideseach of a plurality of indications of impedance, power, voltage level,current level, etc.

Variable resistors R3 and R4, in combination with operational amplifier120, are representative of adjustments to be made to correlate theoutput current (i_(S)) of the galvanic cell with the tissue temperatureof ablation site 20. Calibration circuit 116 can perform theabove-described correlation of the thermistor indicated temperature withthe current output signal of the galvanic cell to obtain a tissuetemperature indication of the ablation site as a function of the current(i_(S)) generated by the galvanic cell. A readout 122, connected viaconductors 124,126 with the calibration circuit, may be employed toprovide an indication of the tissue temperature of the ablation site. Anoutput signal from the calibration circuit is also conveyed viaconductors 124 and 128 to servo amplifier 94. This output signal isreflective of the tissue temperature at the ablation site. Thereby, theservo amplifier receives an input signal reflective of the tissuetemperature at the ablation site. Circuitry of servo amplifier 94 willdetermine whether to raise or lower the tissue temperature of theablation site or to maintain it at its preset temperature. A commandsignal to increase, to decrease, or to maintain the power output of theRF generator is transmitted from servo amplifier 94 through conductor 96to the RF generator.

Referring to FIG. 6, there is illustrated a variant of probe 14 useablewith the present invention. The combination of first mapping a site ofinterest and then ablating the site is a lengthy procedure. Were itpossible to ablate a site identified during a mapping procedure withoutrelocating the probe or without replacing the mapping probe with anablating probe, significant time would be saved. FIG. 6 illustrates acatheter probe 130, which may be sufficiently flexible to position allor some of its length in contacting relationship with the surface of themyocardial tissue to be mapped. A tip 132, which may be similar to tip30 of probe 14, is disposed at the distal end. A plurality of mappingelectrodes, such as rings 134, 136, 138, 140 and 142 are disposedproximally along the probe from tip 132. These rings serve a function ofmapping the tissue of interest to identify and locate a site to beablated to destroy the circuit responsible for errant impulses. Forthese rings to work in the manner of tip 30, as described with referenceto FIGS. 1-5, the rings are preferably metallic and have a work functiondifferent from that of plate (or electrode) 34. One of a plurality ofconductors 144, 146, 148, 150, 152 and 154 interconnect the respectivetip and rings with the output of a switching circuit(s) 160. A dataacquisition circuit 162 is selectively interconnected through switchingcircuit 160 to each of rings 132-142 and possibly tip 132. The dataacquisition circuit collects data sensed by the rings and/or tips to mapthe tissue surface traversed by the probe. Upon detection of a site tobe ablated to destroy an impulse pathway (circuit), switch circuit 160switches to interconnect the respective ring (or tip) with RF generator164. Upon such interconnection, the respective ring (or tip) willirradiate the identified site with RF energy and the ablation function,as described above along with the tissue temperature control function,will be performed.

From this description, it is evident that upon detection of a sitelocated by performing a mapping function, ablation of the site can beperformed immediately without further movement or manipulation of thecatheter probe. Furthermore, the ablation function can be performed withthe circuitry illustrated in FIG. 5 to heat and maintain the tissue at apredetermined temperature until ablation is completed.

Empirically, it has been determined that the circuit and apparatus forablating tissue, as illustrated in FIG. 5, provides to a physician avery accurate indication of the tissue temperature at the ablation site.With such accuracy, ablation procedures are capable of being performedon thin wall tissue without fear of coagulation of the tip, adhesion oftissue to the tip or puncture, which fears exist with presently usedablation performing apparatus. Furthermore, accurate representation ofthe temperature at the ablation site is no longer critically dependentupon the orientation of the probe at the ablation site nor upon theextent of the depression of the tissue in response to the pressureexerted by the probe tip. Because of these very difficult to controlvariables, complete ablation of the errant impulse pathway was notalways achieved if the physician were overly cautious. Tip coagulation,sticking tissue and sometimes excessive injury to and puncture of thetissue occurred if the physician were more aggressive. These resultswere primarily due to the inaccuracy of the information conveyed to thephysician during the procedure and not so much due to poor technique.

As will become evident from the above description, tip 30 (and tip 132)does not need a thermistor or a thermocouple to set or determine thetemperature of the ablation site. Therefore, the probe can be smallerand more versatile than existing probes. Moreover, the probe can bemanufactured at a substantially reduced cost because it is more simplethan existing devices.

Rings (or other electrodes) located on the catheter can be used formapping sites of errant impulses and any of the rings (or otherelectrodes) can be used to irradiate the tissue at such site afteridentification of the site and without repositioning of the catheter.

As a result of in vivo testing on canines in conjunction with moreaccurate and expanded signal displays, a further important capability ofthe present invention has been uncovered. Referring to FIG. 7A, there isillustrated a graph of three signals present during an ablationprocedure. The ordinate of the graph depicts time in seconds and theabscissa depicts voltage. Curve 170 depicts the RF power level appliedto catheter tip 30 and the voltage scale is proportional to the powerlevel. The power applied is shown as steps 172, 174 and 176. The poweris maintained essentially constant at each of these power levels. Thepower is turned on at time T₁ and turned off at time T₂. Curve 180depicts the output of the thermistor within tip 30 (such as thermistor66 within tip 60 shown in FIG. 3) and the voltage scale is proportionalto the temperature sensed by the thermistor. Prior to time T₁, section182 of curve 180 is essentially quiescent and representative of anessentially constant temperature. Upon application of power, thetemperature recorded by the thermistor increases, as depicted by section184, which increase is essentially correlated with the time of powerlevel 172. Upon further increase of the power level (174) section 186depicts a higher temperature. Similarly, upon application of power level176, section 188 depicts a yet higher temperature level. Aftertermination of the power applied at time T₂, the temperature of thethermistor drops, as depicted by section 189.

The current (I_(O)) generated by the galvanic cell is represented bycurve 190 and the voltage scale is proportional to the current. Prior totime T₁, the current is essentially constant, as depicted by segment192. At time T₁ and upon application of RF power, the current increases,as depicted by segment 194, until a quiescent state is established afteran initial duration of applied power corresponding with power level 172.Upon an increase of applied power level 174, the current increasessharply in segment 196. During the latter time period of power level174, the rate of increase of current during segment 196 decreases. Uponapplication of additional power, represented by power level 176, therate of increase of current level depicted by segment 198 remainsessentially constant to a peak identified by numeral 200. It is to benoted that this peak occurs after power corresponding with power level176 has been applied for a short duration. Thereafter, the currentsteadily decreases (decays). It may be noted that the peak of the curverepresenting the temperature of the thermistor, and depicted by point188A, occurred significantly later than the peaking of curve 190 atpoint 200.

The cause for peaking of the current produced by the galvanic cellduring application of a constant power level was not immediatelyunderstood nor evident from the data. Upon further inspection of an invivo ablation site in the heart of a canine, it was learned that peakingoccurred simultaneously with tissue damage (discoloration) at theinterface between the catheter tip and the tissue. It is believed thatthe tissue damage resulted in a change in ion and cation distribution,or change in charge distribution, at the ablation site. That is, theresulting environment of damaged tissue having a reduced chargedistribution significantly affected the current generated by thegalvanic cell and provided a clear and unambiguous signal.

From these results, one can then draw the following conclusions. First,and as set forth above, the output current of the galvanic cell iscorrelatable as a function of the temperature of an ablation siteirradiated with RF energy. Second, the current output of the galvaniccell formed by the subject undergoing an ablation procedure provides anunambiguous and readily apparent indication (signal) of when the tissuesought to be ablated at an ablation site has in fact been ablated.Third, upon detection of peak 200 during an ablation procedure, furtherapplication of RF power may be terminated. Since ablation generallyrequires a temperature in the range of about 50 to 55 degreesCentigrade, conditions giving rise to tip coagulation, sticking tissueand perforation of the tissue will not occur. The resulting safetyfeature of the ablation procedure and the elimination of seriouspossibility for consequential injury will be achieved to a degree neverbefore available.

Referring to FIG. 7B, there is representatively shown a curve 210depicting applied RF power levels, curve 212 depicting the temperatureof a thermistor disposed in a catheter tip performing an ablationprocedure, and curve 214 depicting the current output of a galvanic cellwhich would be present during an ablation procedure. Curve 214 depicts apeak 216 occurring during application of power corresponding with powerlevel 218. At this power level, segment 220 of curve 212 has an initialrise followed by a reduced rate of rise of temperature. Despite theconstantly applied power level, curve 214 decreases subsequent to peak216. Upon application of a higher power level, represented by numeral220, the decrease of curve 214 is halted and after a small risemaintains an essentially quiescent state. However, segment 224 of curve212 increases abruptly with a following reduced rate of increase. Upontermination of power at T₂ curves 214 and 212 decrease.

The curves depicted in FIG. 7B clearly show that peak 216 occurring incurve 214 is unaffected by subsequent applications of increased powerand despite such increased power provides an unambiguous indication ofablation of the tissue at an ablation site.

It is presently believed that the degree of decay of the current signal(curve 180 or 214) is a function of the tissue damage. Moreover, it isbelieved that the depth of ablation can be controlled as a function ofpower level and time subsequent to occurrence of ablation (peak 200 or216).

Referring to FIG. 8, there is illustrated an improved version of theapparatus shown in FIG. 5. The improved version includes a computer 250which includes a visually perceivable display screen 251 for depictingdata, two-dimensional images, etc. For example, readout 123 (depicted inFIG. 5) may be one of the group of images that would be displayed bycomputer 250. The computer may include a plurality of ports, representedby block 252, through which data, whether digital or analog, may beinputted and outputted. Load/impedance measurement circuit 118 isconnected to a port 254 of block 252 via conductor 256. The computer 250includes the capability for manually or otherwise inputting data thatwould affect the parameters, operation, or results achieved during anablation procedure. A port 258 will provide, through conductor 260, anon/off switching function for RF generator 98. A reference voltagerepresentative of a temperature can be applied to servo amplifier 94through conductor 262 via port 264. The readout function formerlyperformed by readout 122 (see FIG. 5) can be provided by computer 250 byinterconnecting conductor 266 via port 268. Furthermore, the curvesdisplayed in FIGS. 7A and 7B may be readily displayed by computer 250through use of its display screen 251.

With the use of a computer and associated software, it is now possiblefor a surgeon to determine on a real time basis the exact momentablation occurs at an ablation site by denoting the presence of peak 200(FIG. 7A) or peak 216 (FIG. 7B). Thereafter, further application of RFpower is unnecessary and all of the potential hazards of overheating atthe ablation site are avoided. However, as the depth of ablation in thetissue is or may be a function of the power level per time of applied RFpower, radiation of RF energy may be continued until the level ofablation desired by the surgeon is achieved.

As discussed above, a catheter tip having multiple elements, as depictedin FIG. 6, can be used to simultaneously or sequentially ablate each ofa plurality of sites. The use of a computer 250 permits real timemonitoring of each ablation site. With such monitoring, control of RFpower applied to each ablation site is readily available to a physician.

While the invention has been described with reference to severalparticular embodiments thereof, those skilled in the art will be able tomake the various modifications to the described embodiments of theinvention without departing from the true spirit and scope of theinvention. It is intended that all combinations of elements and stepswhich perform substantially the same function in substantially the sameway to achieve the same result are within the scope of the invention.

What is claimed is:
 1. Apparatus for sensing ablation of tissue at anablation site during a tissue ablation procedure, said apparatuscomprising in combination:(a) a catheter having a tip for irradiatingthe ablation site with RF energy to perform the ablation procedure; (b)a source of RF energy for transmitting RF energy to said tip; (c) anelectrode adapted to be adjacent tissue; (d) a galvanic cell formed bysaid tip, said electrode and the tissue for generating a uniqueelectrical signal responsive to ablation of the tissue at the ablationsite; and (e) a control circuit responsive to the unique electricalsignal for controlling the RF energy transmitted to said tip.
 2. Amethod for sensing ablation of tissue at an ablation site during atissue ablation procedure, said method comprising the steps of:(a)transmitting RF energy to a tip from a source of RF energy; (b)irradiating the ablation site with RF energy from the tip to perform theablation procedure; (c) locating an electrode adjacent tissue; (d)generating an electrical signal responsive to ablation of the tissue atthe ablation site with a galvanic cell formed by the tip, the electrodeand the tissue; and (e) controlling the RF energy transmitted to the tipwith a control circuit responsive to the electrical signal.
 3. Apparatusfor sensing ablation of tissue at an ablation site during a tissueablation procedure, said apparatus comprising in combination:(a) acatheter having a tip for irradiating the ablation site with RF energyto perform the ablation procedure; (b) a source of RF energy fortransmitting RF energy to said tip; (c) an electrode adapted to beadjacent the tissue; (d) a generator for generating an electrical signalresponsive to ablation of the tissue at the ablation site, saidgenerator comprising said tip, said electrode and the tissue; and (e) acontrol circuit for controlling the RF energy transmitted to said tip asa function of ablation of the tissue and in response to the electricalsignal.
 4. A method for sensing the occurrence of ablation of tissue atan ablation site during a tissue ablation procedure, said methodcomprising the steps of:(a) transmitting RF energy from a source of RFenergy to a tip; (b) irradiating the ablation site with RF energy fromthe tip to perform the ablation procedure; (c) locating an electrodeadjacent the tissue; (d) generating an electrical signal responsive toablation of the tissue at the ablation site with a generator formed bythe tip, the electrode and the tissue; and (e) controlling the RF energytransmitted to the tip with a control circuit as a function of theablation of the tissue and responsive to the electrical signal. 5.Apparatus for sensing ablation of tissue at an ablation site during atissue ablation procedure, said apparatus comprising in combination:(a)a first electrode for irradiating the ablation site with RF energy toperform the ablation procedure; (b) a source of RF energy fortransmitting RF energy to said first electrode; (c) a second electrodeadapted to be adjacent tissue; (d) a galvanic cell formed by said firstand second electrodes and the tissue for generating a unique electricalsignal responsive to ablation of the tissue at the ablation site; and(e) a control circuit responsive to the unique electrical signal forcontrolling the RF energy transmitted to said first electrode.
 6. Amethod for sensing ablation of tissue at an ablation site during atissue ablation procedure, said method comprising the steps of:(a)transmitting RF energy from a source of RF energy to a first electrode;(b) irradiating the ablation site with RF energy from the firstelectrode to perform the ablation procedure; (c) locating a secondelectrode adjacent tissue; (d) generating an electrical signalresponsive to ablation of the tissue at the ablation site with agenerator formed by the first and second electrodes and the tissue; and(e) controlling the RF energy transmitted to the first electrode with acontrol circuit responsive to the electrical signal.
 7. Apparatus forsensing the extent of ablation of tissue at an ablation site during atissue ablation procedure, said apparatus comprising in combination:(a)a catheter having an electrode for irradiating the ablation site with RFenergy to perform the ablation procedure; (b) a source of RF energy fortransmitting RF energy to said electrode; (c) a further electrodeadapted to be adjacent the tissue; (d) a generator for generating anelectrical signal responsive to ablation of the tissue at the ablationsite, said generator comprising said electrode, said further electrodeand the tissue; and (e) a control circuit for controlling the RF energytransmitted to said electrode as a function of ablation of the tissueand in response to the electrical signal.
 8. A method for sensing theextent of ablation of tissue at an ablation site occurring during atissue ablation procedure, said method comprising the steps of:(a)transmitting RF energy from a source of RF energy to a first electrode;(b) irradiating the ablation site with RF energy from the firstelectrode to perform the ablation procedure; (c) locating a secondelectrode adjacent the tissue; (d) generating an electrical signalresponsive to ablation of the tissue at the ablation site with agenerator comprising the first and second electrodes and the tissue; and(e) controlling the RF energy transmitted to the first electrode with acontrol circuit as a function of the ablation of the tissue andresponsive to the electrical signal.